FDV off frequency ??

Peter Bentley

When I transmit on 14,236.000 MHz USB, the actual transmit envelope is between 14,23680  and 14,23820 MHz ??? is this normal  ??

Looks like a lot of fun

Thank you FlexRadio

G4BIM

pa0bie

That is correct.

Steve-N5AC

Peter, this is correct.  The carrier frequency is 14,236 MHz but there is no carrier of course.  Because some radios have issues (typically from filters) effectively reproducing frequencies below 300Hz, the modem avoids these frequencies and sends them above that frequency.

K2CM

Make sure XIT is turned off, then try again. My F6700 's transmit envelope it right where it should be. Note that the Flex was not designed to analyze itself.

Peter Bentley

Thanks for that Steve

I notice the Low cut and High Cut are pre set at 300 to 3000Hz and the transmit width seems to be about 1.4 kHz.

Is it possible that later we might widen the bandwidth to gain an audio bandwidth of say 4.5kHz whilst only using around 2kHz of actual RF band space.

Kind regards

G4BIM  

KY6LA_Howard

You can set your TX and RX bandwidth to whatever you wish... Those default presets work well...

Peter Bentley

Hello Howard

Thank you for your comment

Unfortunately I am unable to monitor or decode the received audio from my 6700, but I can view its transmit waveform on my 5000A.

The default setting of 300 to 3000Hz gives an RF TX bandwidth of around 1400Hz. This does not change until you reduce the TX audio high cut setting down to 2100Hz and if you lower it more the TX bandwidth narrows even further.

However, if you increase the High cut to 4500 hz or more, the RF TX bandwith does not widen beyond 1400Hz.

Are you saying that the transmitted hence received audio, now has a 4500Hz audio bandwidth without an apparent increase in the transmitted RF bandwith ??

Kind regards

Peter G4BIM

Steve-N5AC

You can read all about the details on the FreeDV site I'm sure, but briefly let me explain how this works and the relationship between audio and RF bandwidth.  On transmit, the audio from the microphone goes to the CODEC2 codec.  This takes the individual speech sounds and turns them into a series of tokens that represent the speech.  At this point there is a stream of ones and zeros that must be transmitted.  This is where the modem comes into play.  The modem takes the ones and zeros and turns this into a series of carriers.  The audio bandwidth that is the input to the codec is independent of the RF bandwidth of the modem.  The two are related in that the quality of the audio codec determines the bitrate of the codec output which drives the bandwidth requirement on the RF side, but adjusting the audio filter won't directly affect the modem -- it would have to be placed in a different mode to alter it's bandwidth.  Of course all this happens in reverse in the receiver.

Peter Bentley

Thank you Steve, as ever a very helpful explanation

Kind regards

G4BIM